A reef is a bar of rock, sand, coral or similar material, lying beneath the surface of water. Reefs might be up to 261 feet (80 m) below the surface.
Many reefs result from abiotic processes—deposition of sand, wave erosion planing down rock outcrops, and additional natural processes—but the best-known reefs are the coral reefs of tropical waters developed through biotic processes dominated by corals and calcareous algae. Artificial reefs such as shipwrecks are at times created to enhance physical complexity on generally featureless sand bottoms in order to attract a diverse assemblage of organisms, especially fish.
There is a variety of biotic reef types, including oyster reefs, but the most massive and widely distributed are tropical coral reefs. Although corals are major contributors to the framework and bulk material comprising a coral reef, the organisms most responsible for reef growth against the constant assault from ocean waves are calcareous algae, especially, although not entirely, species of coralline algae.
These biotic reef types take on additional names depending upon how the reef lies in relation to the land, if any. Reef types include fringing reef, barrier reefs, as well as atolls. A fringing reef is a reef that's attached to an island. A barrier reef forms a calcareous barrier around an island resulting in a lagoon between the shore and the reef. An atoll is a ring reef with no land present. The reef front (ocean side) is a high energy locale whereas the internal lagoon will be at a lower energy with fine grained sediments.
One useful definition distinguishes reefs from mounds as follows. Both are considered to be varieties of organosedimentary buildups: sedimentary features, built by the interaction of organisms and their environment, that have synoptic relief and whose biotic composition differs from that found on and beneath the surrounding sea floor. Reefs are held up by a macroscopic skeletal framework. Coral reefs are an excellent example of this kind. Corals and calcareous algae grow on top of one another and form a three-dimensional framework that's modified in various ways by additional organisms and inorganic processes. By contrast, mounds lack a macroscopic skeletal framework. Mounds are built by microorganisms or by organisms that don't grow a skeletal framework. A microbial mound might be built exclusively or primarily by cyanobacteria. Excellent examples of biostromes formed by cyanobacteria occur in the Great Salt Lake of Utah (USA), and in Shark Bay, Western Australia.
Cyanobacteria don't have skeletons and individuals are microscopic. Cyanobacteria encourage the precipitation or accumulation of calcium carbonate and can produce distinct sediment bodies in composition that have relief on the seafloor. Cyanobacterial mounds were most abundant before the evolution of shelly macroscopic organisms, but they still exist today (stromatolites are microbial mounds with a laminated internal structure). Bryozoans and crinoids, common contributors to marine sediments throughout the Mississippian (for example), produced a quite different kind of mound. Bryozoans are small and the skeletons of crinoids disintegrate. Notwithstanding bryozoan and crinoid meadows can persist over time and produce compositionally distinct bodies of sediment with depositional relief.
Geologic reef structures
Ancient reefs buried within stratigraphic sections are of considerable interest to geologists because they provide paleo-environmental information about the location in Earth's history. In addition, reef structures within a sequence of sedimentary rocks provide a discontinuity which might serve as a trap or conduit for fossil fuels or mineralizing fluids to form petroleum or ore deposits.
Corals, including a few major extinct groups Rugosa and Tabulata, have been important reef builders through much of the Phanerozoic after the Ordovician Period. Notwithstanding additional organism groups, such as calcifying algae, especially members of the red algae Rhodophyta, and molluscs (especially the rudist bivalves throughout the Cretaceous Period) have created massive structures at various times. During the Cambrian Period, the conical or tubular skeletons of Archaeocyatha, an extinct group of uncertain affinities (possibly sponges), built reefs. Other groups, such as the Bryozoa have been important interstitial organisms, living between the framework builders. The corals which build reefs today, the Scleractinia, arose after the Permian–Triassic extinction event that wiped out the earlier rugose corals (as well as a large number of additional groups), and became increasingly important reef builders throughout the Mesozoic Era. They might have arisen from a rugose coral ancestor. Rugose corals built their skeletons of calcite and have a different symmetry from that of the scleractinian corals, whose skeletons are aragonite. Notwithstanding there are a few unusual examples of well-preserved aragonitic rugose corals in the late Permian. In addition, calcite has been reported in the initial post-larval calcification in a few scleractinian corals. Nevertheless, scleractinian corals (which arose in the middle Triassic) might have arisen from a non-calcifying ancestor independent of the rugosan corals (which disappeared in the late Permian).